Climate does change over time. We know this because we can directly study aspects of climate such as temperature and greenhouse gas concentration in modern and historical times, and we can use indirect methods to examine changes over longer time periods. The graphs below illustrate changes in climate elements over very long time scales (hundreds of thousands to millions of years).

The first two graphs show changes in oxygen isotope ratios that are derived from deep-ocean limestone cores. Oxygen exists in three forms, or isotopes, the most common of which are oxygen-16 and oxygen-18, and the ratio of these two forms of oxygen in ocean sediments is connected to the ocean’s temperature.

Limestone sediments form from the calcite shells of marine microorganisms that die and sink to the ocean floor. The calcite in the shells, and thus in the limestone, contains oxygen taken from the surrounding ocean water during shell formation. As layers of calcite are deposited on the ocean floor and become limestone, the oxygen is trapped inside the developing rock in the same ratio as it was in the water when the shell formed, millions of years ago. Scientists can analyze the rock to determine how much oxygen-18 and oxygen-16 are present relative to one another, which tells them what the temperature was like when the calcite formed. A higher ratio of oxygen-18 to oxygen-16 indicates that the water was cooler when the calcite was formed.

In the graph above, the dotted line shows average modern temperature as measured in 1950. The right axis indicates values of oxygen isotope O-18 as measured in deep sea carbonate/calcite sediment cores. Measures of temperature can be inferred from these values, and the left axis has estimated values for temperature at Vostok, Antarctica, near the geomagnetic South pole. This graph illustrates that temperature has not been constant at this location during the last 5.5 million years.

This graph shows fluctuating global temperatures and oxygen isotope levels through the Phanerozoic eon (from roughly 543 million years ago - the Cambrian Explosion - to the present). As you can see, temperature is anything but stable!

This graph shows changes in atmospheric CO2 concentration over the past 400,000 years as measured in ice cores. This technique is similar to measuring oxygen levels in sediment cores; atmospheric CO2 becomes incorporated into ice as it freezes, and cores of ice can be extracted from glaciers and ice sheets and their gaseous content analyzed to obtain levels of atmospheric CO2 at various times in the past.

The inset in this graph details changes in CO2 concentration over the last 1000 years; a clear and rapid increase is apparent starting in the 1800s, the start of the Industrial Revolution.